1. Field of the Invention
The present invention relates to a method of producing a semiconductor substrate, and particularly to a method of producing a semiconductor substrate which is dielectrically separated or formed in a single-crystal semiconductor layer on an insulator so as to be suitable for electronic devices and integrated circuits.
2. Description of Prior Art
Formation of a single-crystal semiconductor layer on an insulator is widely known as "silicon on insulator (SOI) technique" and is investigated in various fields because devices formed by employing the SOI process have many advantages which cannot be attained by general bulk Si substrates used for forming Si integrated circuits. Namely, the use of the SOI technique permits the attainment of the following advantages:
1. It is easy to perform dielectric separation and possible to perform high integration.
2. The radiation resistance is excellent.
3. It is possible to reduce the floating capacity and increase the operating speed.
4. It is possible to omit the well process.
5. It is possible to prevent latching-up.
6. It is possible to form a fully depletion-type field effect transistor by reducing the thickness.
Methods of forming SOI structures have been investigated for several years with a view to realizing the above-described characteristic advantages of these devices. The contents of the investigation are summarized in, for example, the following document: Special Issue: "Single-crystal silicon on non-single-crystal insulators"; edited by G. W. Cullen, Journal of Crystal Growth, volume 63, no 3, pp 429-590 (1983).
The SOS (silicon on sapphire) technique for hetero-epitaxy of a Si layer on a single-crystal sapphire substrate by CVD (chemical vapor deposition) has also been known for a long time. Although the SOS technique is successfully achieved as the most mature SOI technique for the present, the SOS technique has the problem that large quantities of crystal defects occur due to the lattice non-conformity at the interface between the Si layer formed and the ground sapphire substrate and that aluminum is mixed in the Si layer from the sapphire substrate. First of all, the high price of the substrate and the retardation in increase in the area inhibit the widening of the application of the SOS technique.
In relatively recent years, attempts have been made to realize a SOI structure without using a sapphire substrate. Such attempts are roughly divided into the following two types:
1. After the surface of a Si single crystal substrate has been oxidized, a window is formed in the oxide surface to partially expose the Si substrate, and a Si single crystal layer is formed on SiO.sub.2 by lateral epitaxial growth using as a seed the window (in this case, the Si layer is deposited on SiO.sub.2).
2. A Si single crystal substrate is used as an active layer so that SiO.sub.2 is formed below the substrate (in this case, no Si layer is deposited on SiO.sub.2).
Known means for realizing the above method 1 include a method of epitaxially growing a Si single-crystal layer in the lateral direction directly by the CVD process, a method of depositing amorphous Si and then epitaxially growing a solid phase in the lateral direction, a method of applying a convergent energy beam such as an electron beam, a laser or the like to an amorphous or polycrystal Si layer to grow a single crystal layer on SiO.sub.2 by melting recrystallization, and a method of scanning a zone melt region by using a rod-shaped heater (zone melting recrystallization). Although these methods have advantages and disadvantages, they have not yet been yet put into practical use in the industrial field because they may have problems with respect to controllability, productivity, uniformity and quality. For example, the CVD process requires sacrificial oxidation for forming a flat thin film. The solid growth method produces a crystal having defective crystallinity. The beam annealing method using an energy beam has the problems with respect to the processing time required for scanning by using a convergent beam and controllability of the degree of overlap of beams, focusing and so on. Although the zone melting recrystallization method among the above methods is most mature, and a relatively large scale integrated circuit can be experimentally formed by this method, many crystal defects such as sub-grains and the like still remain, and no minority carrier device can be formed by this method. Any one of these methods requires a Si substrate and thus cannot form a Si single crystal layer of high quality on a transparent amorphous insulating substrate such as a glass substrate.
The above method 2 in which a Si substrate is not used as a seed for epitaxial growth includes the following four methods:
1. An oxide film is formed on a Si single crystal substrate having a surface with V-shaped grooves formed therein by anisotropic etching, a polycrystal Si layer is deposited on the oxide film so that the thickness is substantially the same as that of the Si substrate, and a Si single crystal region surrounded by the V-shaped grooves so as to be dielectrically separated is then formed by grinding from the rear side of the Si substrate. Although this method produces a layer having good crystallinity, it still has problems with respect to the process of depositing polycrystal Si having a thickness as large as several hundreds microns and the process of leaving only a separate Si active layer on the substrate by grinding the rear side of the Si single crystal substrate, and problems involving controllability and productivity.
2. A method called SIMOX (separation by ion implanted oxygen) in which a SiO.sub.2 layer is formed by implanting oxygen ions in a Si single crystal substrate. This method at present is the most mature process because of its good conformity with the Si process. However, it is necessary for forming the SiO.sub.2 layer to implant oxygen ions in an amount of 10.sup.18 ions/cm.sup.2 or more. The implantation of ions takes much time, and thus it cannot be said that the method has high productivity. In addition, the wafer is high in cost, and many crystal defects still remain. The SiO.sub.2 layer has quality insufficient to the formation of a minority carrier device.
3. A method of bonding a Si single crystal substrate to a separate Si single crystal substrate or quartz substrate which is subjected to thermal oxidation by heat treatment or using an adhesive to form a SOI structure. In this method, it is necessary for forming a device to form a uniform thin active layer. Namely, it is necessary to grind the Si single crystal substrate having a thickness of several hundreds microns to a thickness on the order of one micron or less. The method therefore has many problems with respect to its productivity, controllability and uniformity. In addition, the need for two substrates causes an increase in the cost.
4. A method of forming a SOI structure by dielectric separation caused by oxidation of porous Si. In this method, an n-type Si layer is formed in an island-like shape on a surface of a p-type Si single crystal substrate by implanting proton ions (Imai et al., J. Crystal Growth, vol 63, 547 (1983)) or epitaxial growth and patterning, only the p-type Si substrate is made porous by anodic etching in a HF solution in such a manner that the Si island on the surface is surrounded by the solution, and the n-type Si island is then dielectrically separated by enhanced oxidation. This method has the problem that the degree of freedom for design of a device is in some cases limited because the Si region separated is determined before the device process.
The thin film Si layer deposited on a glass substrate representative of light-transmitting substrates is generally an amorphous layer or, at best, a polycrystal layer because the Si layer reflects the disorder of the crystal structure of the substrate, and no high-quality device can thus be formed by using the Si layer. This is because the substrate has an amorphous crystal structure, and the fact is that a single crystal layer of high quality cannot be easily obtained by simply depositing a Si layer.
The formation of a semiconductor device on a light-transmitting substrate is important for forming a contact sensor and a projection-type liquid crystal image display, which serve as light-receiving devices. In addition, a high-quality driving element is required for further increasing the density, resolution and fineness of pixels (picture elements) of such a sensor or display. It is consequently necessary to produce an element to be provided on a light-transmitting substrate by using a single crystal layer having excellent crystallinity.
It is therefore difficult to produce a driving element having properties sufficient for the present demands or future demands because the crystal structure of an amorphous Si or polycrystal Si has many defects.
However, any one of the methods using a Si single crystal substrate is unsuitable for obtaining a good single crystal film on a light-transmitting substrate.
In addition, since the rate of thermal oxidation of a Si single crystal is about 1 micron per hour (wet oxidation at 1200.degree. C. and atmospheric pressure), several hundreds of hours are required for oxidizing a whole Si wafer having a thickness of several hundred microns and leaving the surface layer unoxidized. Further, it is known that when Si is oxidized to SiO.sub.2, there is an accompanying increase in volume by 2.2 times. This sometimes causes the problem that, if a Si substrate is oxidized without any other processing, cracks or warpages may occur in the Si layer owing to the application of stress exceeding the elastic limit to the Si layer remaining on the surface.